(1) Field of the Invention
The present invention generally relates to a mounting method of a semiconductor device, and more particularly to a method of mounting a semiconductor device on a board in accordance with a COB (Chip On Board) method.
(2) Description of the Related Art
V various methods have been proposed as the COB (Chip On Board) method of mounting a semiconductor device on a board, based on purposes and uses of the semiconductor device. A flip-chip mounting method is one of the methods proposed as the COB method. In this mounting method, a semiconductor device (a semiconductor chip) is directly mounted on a board without wires connecting the semiconductor device to the board. The flip-chip mounting method is also called a wireless bonding mounting method.
A description will be given, with reference to FIGS. 1A through 1F, of the flip-chip mounting method.
Pads 2, which are electrodes, are formed on a chip 1 (the semiconductor device) to be mounted on a board 3. Pads 4 which are parts of conductive wiring patterns are formed on the board 3 on which the chip 1 is to be mounted.
First, bumps are formed as shown in FIG. 1A. Referring to FIG. 1A, an end portion of a gold wire 5 is pressed on a pad 2 of the chip 1 and heated by a boding tool so as to be joined to the pad 2. In this state, the gold wire 5 is then removed. As a result, a tear-drop shaped bump 6 is formed on the pad 2. On all the pads 2 of the chip 1, tear-drop shaped bumps 6 are formed in the same manner as that describe above.
Next, the tear-drop shaped bumps 6 are flattened as shown in FIG. 1B. Referring to FIG. 1B, the tear-drop shaped bumps 6 are pressed on a flat plate 7 so that only a point end portion of each of the tear-drop shaped bumps 6 is subjected to the plastic deformation. As a result, the tear-drop shaped bumps 6 are shaped into bumps 6 having substantially the same height.
Conductive paste is then transferred to a surface of each of the bumps 6 as shown in FIGS. 1C and 1D. That is, the end portions of the bumps 6 are immersed in a layer of conductive paste 8 as shown in FIG. 1C and then pulled up therefrom as shown in FIG. 1D. As a result, a drop of the conductive paste 8 is adhered to the end portion of each of the bumps 6. The conductive paste 8 is made, for example, of epoxy resin in which a large amount of silver fillers are distributed. Due to the drop of the conductive paste 8, positive electrical conductivity can be maintained between each of the bumps 6 of the chip 1 and a corresponding one of the pads 4 of the board 3 when the chip 1 is mounted on the board 3.
Next, adhesive 9 is applied to or printed on the surface of the board 3 so that the pads 4 are covered with the adhesive 9 as shown in FIG. 1E. A thermosetting insulating adhesive, made of material including epoxy resin as the principal ingredient, is used as the adhesive 9 to be applied to the board 3. In a state where the chip 1 is mounted on the board 3, the space between the chip 1 and the board is filled with the adhesive 9. As a result, the chip 1 and the board 3 are tightly joined to each other. In addition, a connecting portion in which each of the bumps 6 are joined to a corresponding one of the pads 4 is covered with the adhesive 9, so that moisture is prevented from entering the connection portion by the adhesive 9.
Finally, the chip 1 is mounted on the board 3 as shown in FIG. 1F. Referring to FIG. 1F, the chip 1 is positioned so that each of the bumps 6 of the chip 1 corresponds to one of the pads 4 of the board 3. A thermopressing head then presses the chip 1 on the board 3, so that each of the bumps 6 is pressed on a corresponding one of the pads 4 of the board 3. The adhesive 9 and the conductive paste 8 are thus hardened by the heat, so that the chip 1 is completely mounted on the board 3.
The board on which semiconductor devices are mounted is set and used in electronic equipment, such as a personal computer. Due to the heat generated by the semiconductor devices on the board, the interior of such electronic equipment is at a high temperature. Particularly, in a case where a processor operated at a high frequency is included in the semiconductor device, a large amount of heat is generated. On the other hand, in a case where the electronic equipment is not used, that is, a power supply of the electronic equipment is in an off-state, the interior temperature of the electronic equipment decreases to a room temperature.
The interior temperature variation of the electronic equipment affects the connecting portion in which each of the semiconductor devices and the board are connected to each other as follows.
As shown in FIG. 2, due to the temperature variation, the adhesive 9 between the semiconductor device 1 (the chip) and the board 3 is thermally expanded and contracted, so that the volume of the adhesive 9 is varies. Of course, thermal expansion and contraction occurs in the board 3, the semiconductor device 1 and the bumps 6. However the rate of expansion (contraction) thereof is less than that of expansion of the adhesive 9. Thus, in a case where the temperature is increased, the volume of the adhesive 9 is increased and the increase of the volume of the adhesive 9 functions as a force to increase the distance between the board 3 and the semiconductor device. As a result, a contact force of the bumps 6 to the pads 4 of the board 3 is decreased, so that an electric contact resistance between each of the bumps 6 and a corresponding one of the pads 4 is increased.
Further, when the temperature is repeatedly increased and decreased, the electrical contact resistance is successively increased and finally a disconnection may occur between the bumps 6 and the pads 4.
Accordingly, a general object of the present invention is to provide a novel and useful mounting method of a semiconductor device in which the disadvantages of the aforementioned prior art are eliminated.
A specific object of the present invention is to provide a method of mounting a semiconductor device on a board so that even if the volume of adhesive between the semiconductor device and the board is varied by the variation of temperature, an increase of the electrical contact resistance of the semiconductor device to the board can be prevented.
The above objects of the present invention are achieved by a method of mounting a semiconductor device including bumps, on a board having pads, so that each of said bumps is joined to a corresponding one of said pads, an adhesive to be hardened by heat being provided between said semiconductor device and said board, said method comprising the steps of: pressing said bumps of said semiconductor device on said pads of said board; and heating a portion in which each of said bumps and a corresponding one of said pads is in contact with each other, wherein a pressure of said bumps to said pads reaches a predetermined value before a temperature of said adhesive to which heat is supplied in step (b) reaches a hardening temperature at which said adhesive is hardened.
According to the present invention, since the bumps are pressed on the pads with a pressing force of a predetermined value before the adhesive is completely hardened, the bumps can be securely joined to the pads so as to provide a sufficient contact area. Thus, even if the hardened adhesive is expanded and contracted by the variation of temperature, the electrical contact between the bumps and the pads can be maintained.